The modified optical pathway for transmitted light in a low power microscope system is illustrated in Figure 5. The field diaphragm is now positioned in the correct plane to act as an aperture stop to control the numerical aperture and the size and shape of the light cone for the illuminating and image-forming light rays passing through the condenser. The specimen plane is fully illuminated with equiangular cones (as in Kohler illumination), however the filament is now imaged at back lens mount of the objective instead of the back focal plane.